The present invention relates to air conditioning systems of the heat pump type and particularly to an improved dual source air conditioning and heating system which automatically selects in either the heating or cooling mode of operation the most efficient source for meeting the heating or cooling requirements of the system.
It is well known in the art that reversible air conditioning systems of the heat pump type operate by transfer of heat across an outdoor heat exchanger coil to either reject or absorb heat from the ambient air in order to provide cooling or heating to air circulated across an indoor space heat exchanger coil. In systems of this type, however, the heating capacity becomes very poor at low outside temperatures and the system cannot, therefore, supply the increased heating demand. Under these weather conditions, such prior art heat pump systems have found it necessary to include backup heating systems, such as electrical resistance heating, to supply the necessary heat when outside temperatures fall below acceptable levels. Another disadvantage of such prior art heat pump systems is that when the outdoor air temperature approaches freezing, frost build-up on the outside coil reduces the amount of air passing over the coil. As a result, the efficiency of such systems is greatly reduced during cold weather conditions.
The use of water as a source of heat is also well known, but in many instances, sufficient quantities of water are not readily available. It is possible to use a closed loop water system where the water is recirculated if an external source of heat, e.g. solar, is readily available. The closed loop system makes the rejection of heat difficult during the cooling mode operation of the heat pump.
Reversible refrigeration systems have also operated by transfer of heat across a heat exchanger in association with a supply of water wherein heat is discharged or absorbed from the water in a manner similar to air source heat pumps. The heating capacity of such systems likewise becomes very poor as the water temperature decreases. When such water source systems are utilized over long periods of time when the heating demand is great, the temperature of the water will decrease rapidly towards freezing. The water, which is usually heated by means of solar energy, will therefore be unable to supply the increased heating demand of the system. Water source heat pumps have, therefore, found it necessary to utilize back-up heating systems. Such back-up systems have included an outdoor air heat exchanger which operates when the temperature of the water source falls below a preset minimum temperature. Once the water temperature increases to or above this minimum temperature due to additional solar heating, the system will switch back to the water source heat exchanger and deactivate the air-source exchanger. Such systems, however, operate primarily as water source heat pumps, irrespective of the relative temperatures of the water source and ambient air, and therefore, irrespective of which source will provide more efficient operation.
In addition, such prior art heat pumps do not completely isolate the heat exchanger not in use from the active portions of the system since refrigerant may still be permitted to flow into the inactive heat exchanger. Further, refrigerant previously pumped into a heat exchanger will remain there even though the heat exchanger is not subsequently active in the system. This results in a reduction of the total refrigerant charge supplied to active portions of the system and causes the heat pump to operate at reduced efficiency.
In order to avoid the deleterious effects of frost build-up on the outside heat exchanger or coil when the ambient air temperature approaches freezing, prior systems have included various heating mechanisms to melt the ice. One such method has included reversing the cycle of the heat pump (switching to cooling mode of operation) so that the outdoor coil will function as a condenser containing compressed high pressure gas from the compressor to thereby melt the frost build-up.